Lasers are devices that use the principle of amplification of electromagnetic waves by stimulated emission, and the term "laser" is an acronym for "light amplification by stimulated emission of radiation." Lasers can operate in the infrared, visible or ultraviolet regions of the electromagnetic spectrum.
The operation and theory of lasers are well understood by those of ordinary skill in this art. In brief description, a laser is a device that pumps (i.e., adds energy to) a collection of atoms or molecules to bring them to a condition in which more are initially in an excited state than an unexcited state. In this condition, an incident wave of electromagnetic energy will stimulate more emission than it will absorption, resulting in a net amplification of the incident light.
Laser light's most characteristic aspect is its coherence; i.e., laser light is highly monochromatic (single wavelength), directional, and in phase. In contrast, most ordinary light sources such as an ordinary light bulb emit light of many wavelengths in substantially all directions with random phase.
A second characteristic of certain types of lasers is their ability to produce emitted light at very high power levels. For example, lasers in which the stimulated atoms are neodymium (Nd), often present in crystals such as yttrium aluminum garnet ("YAG"), can produce continuous output powers up to several kilowatts. Even higher peak powers can be obtained by other lasers using special techniques such as Q switching and mode locking familiar to those of ordinary skill in this art. Accordingly, lasers are in common use that have sufficient energy to weld, alloy, cut or otherwise induce high temperature reactions in metals and other substances. Less powerful lasers are also used in many other fields of industry, as well as in medicine, in which laser light can be used for both illumination and surgical purposes.
As noted above, certain types of lasers produce their coherent light in portions of the spectrum (particularly infrared) that are outside of the visible portion of the spectrum. As is well understood by those of ordinary skill in a number of arts, the electromagnetic spectrum covers a wide range of energy, and the visible spectrum generally refers to light having a wavelength of between about 400 nanometers (nm) and 750 nm. Infrared light tends to have a longer wavelength than light in the visible spectrum (i.e., more than 750 nm), and ultraviolet light tends to have a shorter wavelength (i.e., less than 400 nm). As is further known to those of ordinary skill in many arts, wavelength is inversely proportional to the frequency of the wave, and frequency is directly proportional to the energy of the event producing the wave. Thus, longer wavelengths represent lower energy transitions, while shorter wavelengths represent higher energy transitions.
As noted above, one particularly useful type of laser is neodymium (Nd) maintained in a YAG crystal matrix, sometimes abbreviated as a "Nd:YAG" laser. Depending upon several factors, such a laser will produce light with a wavelength of about 1064 nm (1.064 microns) which is within the infrared rather than the visible portion of the spectrum.
Working with such an "invisible" laser beam presents problems both at low and high power levels.
First, at any power level, the exact location of a laser beam generally must be known at almost all times. The beam's location with respect to the object that it may be intended to strike is often critical to the process, experiment, or technique being carried out. More importantly, at high power levels, stray beams can cause serious damage to persons and equipment. For example, when dealing with high powered infrared lasers, a reflected beam with as little as 3% or 4% of the original power of the beam is still capable of burning many common objects, and causing serious bodily harm to persons.
Accordingly, lower power invisible beams must be identified, and higher power beams must be both located and controlled ("managed") in some fashion that prevents them from damaging surrounding objects and persons. One aspect of such management is referred to as "dumping" a laser beam and the apparatus used to carry it out is also referred to as a "laser dump" or a "beam dump." The theoretical goal of a beam dump is to absorb the laser light and its associated power and transfer or convert it into another more manageable and less hazardous form.
The most common devices used to locate beams outside of the visible spectrum, particularly for infrared lasers, are beam "cards" or "catchers" which generally consist of a phosphorescent ("phosphor") composition on a card-like substrate. When placed in position generally in front of the infrared laser beam, the phosphor card emits light in the visible region through a phosphorescence mechanism, the basics of which are well understood by those of ordinary skill in this art and will not be discussed further herein. The phosphor cards, however, suffer from a number of problems. First, phosphor compounds tend to "burn in" upon repeated use, and the cards progressively lose their effectiveness. Additionally, a number of the typical phosphor compounds used in such cards need to be regenerated between use. The cards thus must be left in a lighted environment or have their phosphorescence capacity refreshed in some other manner.
At high powers, the disadvantages become even more severe because high power lasers can destroy the phosphor cards relatively quickly.
There thus exists the need for an improved locator card for infrared lasers of both low and high power outputs.
With respect to beam dumping or other types of beam attenuation, a number of techniques are generally attempted. These include bulk absorption, reflective management, neutral density filters, prism polarizers, and Brewster wedges. Each of these suffers various disadvantages with respect to high power laser beams. In particular, and as noted above, some infrared laser beams are so powerful that even if a small percentage is reflected (for example from the window of a beam dump), it can still do serious damage to persons and equipment. Bulk absorption plates generally consist of semitransparent glass which manages the beam by absorbing a portion of the incident light falling on it. At high incident powers, however, bulk absorption plates tend to fail from optical bleaching or outright fracture.
Prisms have similar disadvantages, and in particular, when high powered laser beams pass through the transparent material of the prisms, the localized heating tends to cause spatial distortion in the transmitted beams which can be unacceptable in certain circumstances, particularly where coherence is required after beam management or "attenuation." Generally speaking, such prism polarizers (e.g., Glan-laser prisms) are unsuitable for use at high power levels.
Brewster wedges likewise create a lot of distortion, and in particular circumstances a large number of such wedges must be used and carefully aligned. The process is tedious and expensive.